The present invention relates to filter units for passing signals of a desired frequency band, and to duplexers comprising such filter units.
FIG. 12 is a graph showing the common characteristics of a low-pass filter (hereinafter referred to briefly as xe2x80x9cLPFxe2x80x9d) for passing low frequencies. FIG. 13 is a block diagram showing the internal construction of the low-pass filter. The filter comprises an inductance L1, a capacitance C1 connected in parallel thereto, and capacitances C2, C3 connected between the inductance L1 and the ground. With reference to FIG. 12, fc is a cutoff frequency. The frequency characteristics gently attenuate from the cutoff frequency fc toward higher frequencies and have an attenuation pole (point where the level of passage is minimized) at a frequency of fp1. Thus, the pass level is minimum at fp1. As is known well, the cutoff frequency fc is dependent on the inductance L1 and capacitances C2, C3, and the attenuation pole frequency fp1 on the inductance L1 and capacitance C1.
However, conventional filters have the following problem.
LPFs have frequency characteristics which attenuate gently from the cutoff frequency fc toward higher frequencies. Accordingly, if there is a narrow frequency band wherein signals are to be blocked for the prevention of noise between the cutoff frequency fc and the attenuation pole frequency fp1 as indicated at fp2 in FIG. 12, this frequency band can not be fully attenuated.
To give sharp attenuation characteristics to the LPF to block the passage of signals of the frequency band fp2 in the vicinity of the cutoff frequency fc, it appears feasible to arrange in series many combinations of capacitance C and inductance L connected thereto as shown in FIG. 15 and to provide a plurality of attenuation poles as seen in FIG. 14. Nevertheless, this makes the circuit construction of the LPF complex, rendering the device incorporating the LPF large-sized.
An object of the present invention is to provide a simple construction for blocking signals of a frequency band positioned between a cutoff frequency and an attenuation pole frequency.
The present invention provides a filter unit comprising a band-pass filter 2 for passing signals of a specified frequency band, a SAW filter 3 or dielectric resonator for passing signals of a band around a frequency fp2 positioned between a cutoff frequency fc of the band-pass filter 2 and an attenuation pole frequency fp1 thereof, and an inversion circuit 4 for inverting the phase of the signal passing through the SAW filter 3 or dielectric resonator and superposing the phase-inverted signal on an output signal from the band-pass filter 2.
The SAW filter 3 passes a signal of a band around a frequency fp2 positioned between the cutoff frequency fc of the band-pass filter 2 and the attenuation pole frequency fp1 thereof. This signal has its phase inverted by the inversion circuit 4. Stated more specifically, the signal of the band around the frequency fp2 to be blocked is attenuated as indicated by a chain line in FIG. 4 and superposed on the output signal form the band-pass filter 2. Consequently, the output signal passing through the filter unit has two attenuation poles fp1, fp2, as shown in FIG. 2, sharpening the attenuation characteristics between the cutoff frequency fc and the frequency fp2 to be blocked. Thus, the signal of a frequency band positioned in the vicinity of the cutoff frequency fc can be blocked by a simple arrangement. This serves to reduce the size of the device into which the filter unit is to be incorporated.